The present invention relates to a semiconductor device fabricating method, more specifically, a method for fabricating a semiconductor device including a ferroelectric capacitor.
In the main memory devices of computers, volatile memories, such as dynamic random access memories (DRAM) and static random access memories (SRAM), etc. are used. The volatile memory can hold data while the power source is being supplied, and loses the data when the supply of the source power is stopped. As nonvolatile memories, which, in contrast to the volatile memory devices, do not lose data even when the supply of the source power is stopped, flash memories and ferroelectric memories (FeRAM) are known.
The flash memory includes a floating gate buried in the gate insulating film of an insulated gate field effect transistor (IGFET) and stores charges in the floating gate to memorize information. To write and erase information, the flash memory must flow tunnel currents through the insulating film and requires relatively high voltages.
The ferroelectric memory includes a ferroelectric capacitor having a ferroelectric film sandwiched between a pair of electrodes and uses the hysteresis characteristic of the ferroelectric to memorize information. The ferroelectric capacitor has polarization corresponding to a voltage applied between the electrodes and has spontaneous polarization even after the applied voltage has been removed. When a polarity of the applied voltage is inverted, a polarity of the spontaneous polarization is also inverted. The ferroelectric memory associates a direction of the spontaneous polarization with memorized information to function as a memory device and detects a direction of the spontaneous polarization to read memorized information. The ferroelectric memory is operative at lower voltages than the flash memory and prospectively can write at high speed with saved electric power.
The semiconductor device fabricating process includes steps in which hydrogen is generated, such as steps for growing inter-layer insulating films. The ferroelectric material is easily reduced with hydrogen, and to fabricate ferroelectric memories of good quality, a capacitor protective film which functions as the hydrogen barrier must be formed, covering the ferroelectric film.
Reference 1 (Japanese published unexamined patent application No. 2001-044375) describes that a capacitor protective film of an aluminum oxide film (Al2O3) of a film density of not less than 2.7 g/cm3 is formed, covering the top of the ferroelectric capacitor, whereby the reduction of the ferroelectric film with a reductive gas, such as hydrogen or others, is prevented. In Reference 1, RF sputtering method using an aluminum oxide target is used to thereby form an amorphous aluminum oxide film while suppressing the generation of particles.
However, for the ferroelectric memory of the next generation, it is preferred to form the capacitor protective film by CVD (Chemical Vapor Deposition) method. This is because the film formation by the current sputtering method is inferior to the film formation by CVD method in the step coverage, and the capacitor protective film formed by the former may degrade the coverage of the capacitor protective film in the stacked capacitor structure of the next generation whose aspect ratio will be larger.
Usually, to form aluminum oxide film by CVD method, tri-methyl aluminum (TMA, Al(CH3)3) and water (H2O) as the oxidizer are used, and ALD (Atomic Layer Deposition) method is used.
However, when aluminum oxide film as the capacitor protective film of the ferroelectric memory is actually formed by CVD method using TAM and H2O, the ferroelectric film is degraded which makes it impossible for the ferroelectric memory to function as a ferroelectric memory. This is because when aluminum oxide film is formed by the above-described method in which much H2O is used, hydrogen or water is adsorbed to the ferroelectric film, or hydrogen remains in the aluminum oxide film, and the hydrogen or water causes reduction of the ferroelectric film in thermal processing in later steps.
Reference 2 (Japanese published unexamined patent application No. 2004-193280) discloses a method for forming an aluminum oxide film as the capacitor protective film of the ferroelectric memory by CVD method using TMA and ozone (O3). That is, the use of O3, which is a hydrogen-free oxidizer, prevents the adsorption of hydrogen or water to the ferroelectric film or the residue of hydrogen in the aluminum oxide film. Thus, the activation of the hydrogen in thermal processing in later steps can be suppressed to thereby prevent the degradation of characteristics of the ferroelectric film.
However, even when aluminum oxide film as the capacitor protective film of the ferroelectric memory is formed by CVD method using TMA and O3, the characteristic degradation which is found to be due to the hydrogen or water often takes place in the ferroelectric capacitor.